Synthesis gas containing hydrogen and carbon monoxide is produced for a variety of industrial applications, for example, the production of hydrogen, chemicals and synthetic fuel production. Conventionally, the synthesis gas is produced in a fired reformer in which natural gas and steam is reformed to the synthesis gas in catalyst filled reformer tubes. The endothermic heating requirements for steam methane reforming reactions occurring within the reformer tubes are provided by burners firing into the furnace that are fueled by part of the natural gas. In order to increase the hydrogen content of the synthesis gas, the synthesis gas can be subjected to water-gas shift reactions to react residual steam in the synthesis gas with the carbon monoxide.
Such steam methane reformers are optimized for hydrogen production and typically are fed with a reactant stream containing hydrocarbons and steam at a steam-to-carbon ratio of 1.5 to 3.5, depending on the quantity of carbon dioxide in the reactant stream, to thereby produce the synthesis gas at a hydrogen to carbon monoxide ratio of 3 or higher. This is not optimal for the production of synthesis gas for synthetic fuel production such as in Fisher-Tropsch or methanol synthesis where the hydrogen to carbon monoxide ratio of 1.8 to 2.0 within the synthesis gas is more desirable. Consequently, where synthetic fuel production is a desired use of the synthesis gas, an autothermal reformer is typically used in which the steam-to-carbon ratio of the reactant is typically between 0.5 and 0.6. In such a reactor, oxygen is used to combust part of the feed to create additional steam and heat to reform the hydrocarbons contained in the feed to the synthesis gas. As such, for a large scale installation, an air separation plant may be required to supply the oxygen.
As can be appreciated, conventional methods of producing a synthesis gas such as have been discussed above are expensive and involve complex installations. In order to overcome the complexity and expense of such installations it has been proposed to generate the synthesis gas within reactors that utilize an oxygen transport membrane to supply oxygen and thereby generate the heat necessary to support endothermic heating requirements of the steam methane reforming reactions. A typical oxygen transport membrane has a dense layer that, while being impervious to air or other oxygen containing gas, will transport oxygen ions when subjected to an elevated operational temperature and a difference in oxygen partial pressure across the membrane. This difference in oxygen partial pressure can be produced by compressing the supplied air or from the combustion of hydrocarbons fed to a permeate side of the membrane and supported by permeated oxygen or a combination of the two methods.
For example, in U.S. Pat. Nos. 6,048,472 and 6,110,979; a reactant gas feed is combined with steam. The reactant gas feed can be natural gas, naptha or other hydrocarbon containing gas. This combined feed stream is then heated and introduced into an adiabatic pre-reformer to produce an intermediate stream that contains carbon monoxide, carbon dioxide, steam, hydrogen, and methane. The intermediate stream can be combined with carbon dioxide and steam. The resulting reactant stream is then introduced with air into reactant and oxidant sides, respectively, of an oxygen transport membrane reformer. The oxygen transport membrane reformer has an oxygen transport membrane separating the reactant and oxidant sides of the reformer. The reactant gas reacts with oxygen that has permeated through the oxygen transport membrane to produce a synthesis gas. Preferably a reforming catalyst is applied to at least a portion of the reactant side surface of oxygen transport membrane or packed into the reactant side to promote the reforming reactions.
U.S. Pat. No. 6,114,400 discloses an integrated system in which an oxygen transport membrane reformer is connected to a downstream reactor such as a Fischer-Tropsch reactor to produce a liquid product. In all of these patents the presence of the pre-reforming stage prevents breakdown of higher order hydrocarbons present in the reactant feed stream and the resulting carbon deposition that would otherwise occur had the higher order hydrocarbons been fed directly to the reactor. Such carbon deposition will degrade the reforming catalyst used in connection with the oxygen transport membrane reactor.
U.S. Pat. No. 6,296,686 discloses a reactor in which heat is supplied to an endothermic reforming reaction inside a reaction passage separated from an air passage by an oxygen transport membrane. A reactant gas, for example, methane flows through the reaction passage is combusted with permeated oxygen to provide the heat to support the reforming reaction. Further heat is supplied to the reforming reaction by either combusting a fuel with retentate or a fuel with a second permeate produced by another oxygen transport membrane or within a combustion passage. Alternatively, an oxygen transport membrane can be situated between an air passage and a combustion passage and a barrier is located between the combustion passage and the reaction passage. In such case, the oxygen transport membrane supplies oxygen permeate to support combustion of a fuel in a combustion passage to generate heat that is transferred to the reaction passage.
US Patent Application Serial No. 2008/0302013 discloses a staged reactor system having a sequential arrangement of reactor stages to produce a synthesis gas product. Each of the reactor stages has an oxidant side separated from a reactant side by an oxygen transport membrane. The reactant sides are linked together so that a reactant stream containing methane and steam is introduced into the system and sequentially reacted with oxygen permeating through the membrane to produce a synthesis gas product for use in a downstream reactor such as a Fischer-Tropsch reactor. Catalyst beds can be located within the reactant side of the reactor stages or can be positioned between the reactor stages. Both steam and a reactant gas from a downstream process utilizing the synthesis gas can be introduced into the feed between stages. The presence of the multiple stages allows the temperature within each of the reaction stages to be controlled to prevent the oxygen transport membrane from being degraded and to control the deposition of soot throughout the membrane system.
US Patent Application Serial No. 2006/0029539 discloses other examples of staged reactor systems that can employ oxygen transport membranes in which the air or other oxygen containing stream fed to each of the stages can be controlled to control the temperatures and conversation that can be obtained in producing a synthesis gas.
The problem with all of the above-identified prior art systems is that an oxygen transport membrane will operate at high temperatures of about 900° C. to 1100° C. Where hydrocarbons such as methane and other higher order hydrocarbons are subjected to such temperatures carbon formation will occur. Additionally, where oxygen is supplied by an oxygen transport membrane directly to the reactor, the surface area of the membrane is distributed throughout the reactor. As such, the distribution of oxygen is non-uniform throughout the reactor. In other words, sufficient quantity of oxygen is not generally available at or near the entrance to the reactor. This also results in an aggravated carbon formation problem at the entrance that is especially the case at low steam-to-carbon ratios. In any case, a reactant containing methane and steam will produce a relatively low oxygen flux across the membrane resulting in the membrane area required for such a reactor to be larger and it will add to the expense and complexity in such a reactor or system. Additionally, a steam methane reforming catalyst must be periodically replaced. In prior art reactor designs where the catalyst is employed adjacent to the oxygen transport membrane, catalyst replacement becomes an expensive if not impractical exercise.
The present invention, in one or more aspects, provides a method and apparatus in which the oxygen transport membrane is not directly used to react the steam and methane components of the reactant feed, but rather, to generate the heat required to support endothermic heating requirements of steam methane reforming reactions within a separate reactor, thus overcoming the above-identified problems.